1. Field of the Invention
The present invention relates to a hydraulic pressure control apparatus, which attains an actuating pressure for controlling a clutch or the like from an initial pressure (line pressure) of a working oil that is input to an input port, and to a hydraulic pressure control apparatus having a relief valve, which is capable of being opened if the pressure of the working oil becomes equal to or greater than a predetermined threshold value.
2. Description of the Related Art
As one type of hydraulic pressure control apparatus that is mounted in an automobile for use with a clutch, the hydraulic pressure control apparatus disclosed in Japanese Laid-Open Patent Publication No. 05-345528 is known. The hydraulic pressure control apparatus includes a differential clutch control valve and a solenoid modulator valve that function as pressure regulating valves, and a single linear solenoid-operated valve. An initial pressure (line pressure) of the working oil is reduced to a predetermined pressure by the solenoid modulator valve, and the reduced oil pressure is supplied to the linear solenoid-operated valve.
The linear solenoid-operated valve generates a solenoid pressure based on an electric current that is supplied to the solenoid thereof, and the solenoid pressure is supplied to the differential clutch control valve. Based on the solenoid pressure, the differential clutch control valve converts the line pressure, and the working oil is supplied to the clutch under the converted pressure, which acts as a predetermined clutch pressure (actuating pressure).
According to Japanese Laid-Open Patent Publication No. 05-345528, among the three valves that function as described above, it has been proposed that the differential clutch control valve and the solenoid modulator valve are arranged in parallel facing relation to each other within a valve body, which is made up of a combination of a lower valve body and an upper valve body, whereas the linear solenoid-operated valve is arranged between the two pressure regulating valves, with the axis of the linear solenoid-operated valve extending perpendicularly to the two pressure regulating valves. Further, oil passageways for the working oil are defined in the upper valve body.
In a hydraulic pressure control apparatus having the aforementioned structure, in general, if the oil pressure rises above the predetermined pressure, in order to protect the oil pressure device, a relief valve is provided for allowing the working oil to escape outside of a working oil passage through which the working oil flows. As an example of this type of relief valve, reference may be made to the disclosure of Japanese Laid-Open Patent Publication No. 07-139324.
In the relief valve, a spherical valve element is resiliently biased toward a valve seat by a closing spring. In addition, if the pressure of the working oil that flows through the oil passageway exceeds the resilient force of the closing spring, the spherical valve element is pressed by the working oil together with compressing the closing spring, whereupon the spherical valve element separates away from the valve seat. Accordingly, the relief valve assumes a valve-open state, and as a result, the working oil is introduced into the body of the relief valve.